Hot deformed, hot rolled and cold rolled metal strip often exhibits departures from flatness which need to be eliminated for further working and finally also for the use of the finished strip.
Such departures from flatness can take the form of curvature of the strip in the longitudinal direction, transverse curvature and transverse, central and edge waves or the formation of a camber as a result of differences in the width of the strip. Such strip cambers lead, over long transport distances, to lateral wandering of the strip. To prevent this, guide rolls which can rotate about an axis normal to the plane of the strip are required, and these are associated with substantial investment and are subject to high wear.
The defects referred to can lead to problems and to damage to the strip in its subsequent treatment. For example, in a looping tower or annealing furnace in which there are large free (i.e. unsupported) lengths of strip it can happen that the strip scrapes along guides or fixed plant, or that when a strip is guided along a meandering path sections which are moving in opposite directions ride over one another. This leads to damage to the surface of the strip, which either has to be eliminated by expensive repair operations or makes it impossible to process the strip to a saleable product. This danger is particularly great in the case of stainless steel strip, for which the surface quality requirements are particularly high. Moreover, departures from flatness in highly hardened stainless steel strip are particularly difficult to eliminate.
In order to eliminate departures from flatness it is, for example, known from "Stahl und Eisen", 1986, pp. 1131 to 1127, to pass the strip round several bending rolls under tension so as to subject it to deformation by bending in alternate directions while under tensile stress. The tensile stress is superimposed on the bending of the strip and the resulting straightening is due to the co-operation of changes in shape by bending and tension and is accordingly dependent on the one hand on the bending radius or angle of wrap on the straightening rolls and on the other hand on the tensile load exerted on the strip, which ensures a certain degree of stretching. In straightening by bending and stretching bending radius and tensile load are inversely related insofar as at constant tensile load the bending radius decreases as the strength of the strip material increases. To ensure an adequate bending radius it is therefore necessary to increase the tensile load as the strength of the material increases. To achieve this, the tension rolls on either side of the bending rolls which move the strip through the straightening apparatus require high driving power. This results not only in high capital costs but also in high operating costs, in particular energy costs.